High frequency heating apparatus



W58 c. E. M. TIBBS HIGH FREQUENCY HEATING APPARATUS 4 Sheets-Sheet 1 Filed July 11, 1955 Inventor fir/k/o zler' i WM;

4 Sheets-Sheet 2 C. E. M. TIBBS HIGH FREQUENCY HEATING APPARATUS Aug. 19 1958 Filed July 11, 1955 in en or CfiT/Sfa/J er 771/5 y Attornys S B m T M E a HIGH FREQUENCY HEATING APPARATUS 4 Sheets-Sheet 5 Filed July 11, 1955 Inventor Cir/k/a fier {/71 7755s B MM Filed July 11, 1955 4 Sheets-Sheet 4 Inventor car/SAW 1 was A ttorneyx HIGH FREQUENCY HEATING APPARATUS Christopher E. M. Tibbs, Wokingham, England, assignor to Radio Heaters Limited, Wokingham, England, a British company Application July 11, 1955, Serial No. 521,358

Claims priority, application Great Britain July 16, 1954 8 Claims. (Cl. 307-156) In high frequency dielectric heating equipment transformers are provided by which the output of the tank circuit is transmitted to the heating electrodes.

The present invention provides an improved arrangement of transformer suitable for use in such equipment, for example in pre-heaters for plastic materials which is very compact, and which can be operated at high frequencies, say, 40 megacycles per second, with very large power circulating in the tank circuit. Powers of the order of 1,000 kilovoltamperes can be handled without the need for water cooling in the coil connected in the tank circuit or in the secondary coil connected either to balanced or unbalanced electrodes.

To enable the output from the equipment to be varied without raising and lowering the electrodes, at variable condenser may be connected in series with the secondary or output coil. However, if a conventional design of condenser is used, the inductance of the condenser and its associated leads would be so great as to limit the performance of the equipment. The present invention, therefore, also enables a series condenser to be introduced into the output circuit while hardly increasing the inductance of the circuit at all, the condenser plates being connected directly with the coil.

AAcording to the invention the transformers comprise,

a tank circuit or primary coil consisting of a single turn of metal strip mounted at its four corners on supports, and a secondary or output coil consisting of a single turn of metal strip surrounding the tank circuit coil and threaded through the gap between the pairs of supports. Conveniently these supports consist of two pairs of pillars. The tank coil may be connected to the tank condenser directly or by a pair of metal strips or blocks each of which extends from a pair of pillars supporting one end of the tank circuit coil.

To enable the output to be varied the secondary or output coil may carry at one end a set of condenser plates co-operating with a set of condenser plates mounted on an output terminal, one set being fixed and the other set being movable to vary the amount of overlap between the two sets of plates which form a series output condenser. If the coils are particularly wide, the tank coil may if necessary be supported on more than the four pillars one at each corner, by having additional intermediate pillars to give it rigidity. In such a case it may be desirable to make the output coil of two narrower strips side by side passing between neighbouring pairs of pillars. The output from these coils may then be fed to the same load or to two different loads fed simultaneously. A series condenser may be connected in each of these two output coils.

Two examples of apparatus according to the invention are shown in the accompanying drawings in which:

Figure 1 shows a side elevation of the first form of the apparatus;

Figure 2 shows a the left in Figure 1;

Patented Aug. 19, 1958 Figure 3 is a side elevation of the output coil and condenser of the second form of apparatus;

Figure 4 is a view of the apparatus shown in Figure 3 as seen from the left, and

Figure 5 is a plan view of the apparatus shown in Figure 4.

In Figures 1 and 2 the tank condenser consists of alternate plates 1 and 2 supported respectively on insulators 3 and 4. The plates 1 are connected together by a conducting bracket 5 bent as shown in Figure 1 and carrying a pair of conducting pillars 6 to which one end of the primary or tank coil 7 is attached. This coil consists of a broad metal strip bent into a cylinder of greater width than diameter, as shown in Fig. l, the other end of the strip being attached to a pair of pillars 3 similar to the pillars 6 and carried by a bracket 9 which is connected across the plates 2 as shown in Fig. 2. The whole of this apparatus is housed in the housing indicated at 10, the plates 1 and 2 being situated beneath the work table 11 from which the coil 7 is separated by an insulating panel 12 which carried an output terminal 13. The output coil 14 consists of ametal strip narrower than the coil 7 and bent as shown in Figure 1. The coil embraces the coil 7 and its upper end is attached to the terminal posts 13 while its lower end passes between the pillars 6 and 8 and is attached to the grounded work table 11.

The tank circuit, consisting of the coil '7 and the tank condenser, is connected to the anode of the oscillator valve or other source of high frequency potential by means of an anode blocking condenser which may be fixed to one of the plates 1 of the tank condenser. The connection to the other end of the tank circuit, represented by the plates 2 of the tank condenser, varies according to the type of oscillator circuit which is being used. in use, an electrode is connected to the output terminal 13 and the work to be heated is placed between the worktable 11 and the electrode. The oscillator circuit is then brought into operation and, as a result of the very efficient coupling between the coils 7 and 14, a very high circulating power can be obtained in the electrode circuit.

In the apparatus shown in Figures 3 to 5, the tank condenser and tank coil are similar to those shown in Figures 1 and 2 and are omitted for the sake of clarity. In this apparatus, however, the upper part of the housing 15 contains an output condenser connected in the upper end of the output coil 16, the lower end of which is connected to the grounded housing at 17. The coil is bent as shown to embrace the tank secondary or output coil 16 and its upper end is connected to a transverse conductor bar 18 carried on brackets 19 supported on insulators 20. These brackets support angle pieces 21 through which a shaft 22 runs carrying five condenser plates 23. These plates are rectangular and are locked to move together by a tie rod 24 running through the corners of all the plates 23.. The four fixed plates 25 of the condenser are carried on an insulating block 26 forming the front of the housing and are connected together by a conductor 27 which forms the output terminal of the apparatus. Thus, by rotating the shaft 22, the capacity of the condenser formed by the plates 23 and 25 can be varied.

To adjust the capacity of the output condenser a knob 28 is provided outside the casing 15 on the end of a shaft 2% which carries a worm (not shown) meshing with a worm wheel 30 on a stub shaft 31 coaxial with the shaft 22. The stub shaft 31 drives the shaft 22 through a fiat insulating coupling strip 33 and a further view of the apparatus as .seen from stub shaft 32.

In use, an electrode or work-table is connected to the grounded terminal 17, and the upper electrode is connected either to the terminal 27 or to an alternative terminal which is electrically connected to the terminal 27. The work to be heated is placed between the two electrodes and the apparatus is switched on. The output condenser is then adjusted by the knob 28 until the load is matched to the oscillator circuit. With this arrangement, the variable condenser is connected in series in the output circuit with a minimum additional inductance, and the equipment can be loaded for a wide variety of work material without prior experiments or measurements.

I claim:

1. An output transformer for high frequency dielectric heating equipment comprising a primary coil consisting of a single turn of metal strip, a pair of spaced supports at each end of said strip and a secondary coil consisting of a single turn of metal strip, said secondary coil surrounding said primary coil and being threaded between the two individual supports constituting each of said pairs of supports.

2. An output circuit for high frequency dielectric heating equipment, said circuit comprising an output transformer having a primary consisting of a single turn of metal strip provided with a pair of spaced supports at each of its ends, and a secondary coil consisting of a single turn or" metal strip surrounding said primary coil and being threaded between the two individual supports constituting each of said pairs of supports, in combination with a condenser electrically connected in parallel with the primary coil of said transformer, the opposite plates of said condenser being connected to the supports at opposite ends of said primary coil.

3. An output circuit arrangement for high frequency dielectric heating equipment comprising an output transformer including a primary coil and a secondary coil, said primary and secondary coils each consisting of a single turn of metal strip and one of said coils being arranged within the other, a set of condenser plates mounted on one end of said secondary coil across the width thereof and a second set of condenser plates interleaving said first set and connected together by an output terminal strip, one of said sets of condenser plates being movable to vary the extent to which the plates thereof penetrate into the spaces between the plates of the other set.

4. An output circuit arrangement according to claim 3 including a transverse shaft on which are mounted the movable plates, whereby the overlap between the two sets of condenser plates can be adjusted by rotating the shaft.

5. An output transformer for high frequency dieleci tric heating apparatus comprising a primary coil and a secondary coil, each consisting of part of a single turn of metal strip, one coil being arranged Within the other.

6. An output transformer according to claim 5, in which the width of the primary coil is greater than its diameter.

7. An output transformer according to claim 4 in which one of said coils is connected at at least one end to a pair of spaced supports so as to define therewith an opening through which the other coil passes.

8. An output transformer according to claim 4 in which the secondary coil surrounds the primary coil, each end of the primary coil being mounted on a pair of spaced supports forming electrical connections for the coils and the secondary coil being threaded through the gaps between the individual supports constituting said pairs of supports.

References Cited in the file of this patent UNITED STATES PATENTS 

